Better IV on B!Celica, +ATK -SPD or +HP -RES?

I'd say +atk -spd because she still has 39 speed if you use Royal Sword and she would still have double lion to double

+Atk -Spd, as she appreciates the extra attack for Double Lion. She will still quad slower enemies, and faster ones are less durable so two hits should be enough to finish them off.